Planets and satellites Notes

"What Counts As A Planet And A Satellite?"

1. A planet is a large natural object in space that moves around the Sun along a fixed path.
2. This path is not random; it follows a predictable route controlled by gravity.
3. Planets do not produce their own light but can be seen because they reflect light from the Sun.
4. The motion of planets helps scientists identify and classify them.

The IAU Definition Of A Planet (2006)

1. In 2006, the International Astronomical Union (IAU) defined a planet in our solar system as an object that:
   1. is not so massive that it starts fusion (otherwise it would become a star),
   2. orbits the Sun,
   3. is sufficiently massive that gravity makes it nearly round,
   4. has cleared the neighborhood around its orbit.
2. The last condition explains why Pluto, Ceres, and Eris are classified as dwarf planets rather than full planets: they share their orbital region with many other objects.

The Solar System Has Eight Planets With Very Different Properties

1. In the modern view, the solar system contains eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
2. They can be grouped into:
   1. Terrestrial (rocky) planets: Mercury, Venus, Earth, Mars (smaller, denser, solid surfaces)
   2. Giant planets: Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants)
3. A key pattern is that distances between planets increase strongly as you move outward.

Approximate Sizes And Distances

The table below summarizes typical distances from the Sun and planetary radii.

Orbits and Planetary Motion

How do planets move around the Sun?

1. Planets move along paths called orbits.
2. An orbit is usually elliptical, meaning it is slightly stretched rather than perfectly circular.
3. The Sun lies at one focus of the elliptical orbit.
4. Gravity between the planet and the Sun keeps the planet in its orbit.

Why Are Some Objects Not Called Planets? (Dwarf Planets)

1. Not all objects that orbit the Sun are classified as planets.
2. Some objects are large and spherical but share their orbital region with many other objects.
3. These objects are called dwarf planets.

Gas Giants

1. Gas giants are very large planets.
2. They are mostly made of gases rather than solid rock.
3. They may have a small solid core at the centre.

Planetary Size and Distance Patterns

1. Planet sizes increase dramatically from Earth-sized planets to Jupiter.
2. Distances between planets increase much faster than their sizes.
3. This creates major challenges when trying to draw or model the Solar System.

Using Ratios to Compare Sizes

1. Jupiter’s radius is about 11 times Earth’s radius.
2. The Sun’s radius is about 109 times Earth’s radius.
3. These ratios help compare objects without memorising exact values.

Scale Models and Why Space Is Mostly Empty

1. A scale model keeps all sizes and distances in the same ratio.
2. In the Solar System, sizes and distances differ by enormous factors.
3. If planets are large enough to see, distances become extremely large.
4. If distances fit on a page, planets become tiny dots.

What Is a Satellite?

1. A satellite is an object that orbits another object.
2. A natural satellite is not made by humans.
3. Most planets have at least one natural satellite.

Motion of Satellites

1. Satellites move in orbits due to gravitational attraction.
2. The satellite’s forward motion and gravity together create a stable orbit.
3. Satellite orbits can be circular or elliptical.

Eclipses and Satellites

1. A lunar eclipse occurs when Earth moves between the Sun and the Moon.
2. Earth blocks sunlight, causing its shadow to fall on the Moon.
3. The curved edge of Earth’s shadow provides evidence that Earth is spherical.

Exoplanets (Planets Beyond the Solar System)

1. Not all planets orbit the Sun.
2. Planets that orbit other stars are called exoplanets.
3. Exoplanets are detected using indirect methods, such as changes in starlight.

Planetary Size and Distance Patterns

1. Planet sizes increase dramatically from Earth-sized planets to Jupiter.
2. Distances between planets increase much faster than their sizes.
3. This creates major challenges when trying to draw or model the Solar System.

How Gravity Shapes Orbits And Planetary Motion

1. Planets orbit because of the gravitational attraction between the Sun and each planet.
2. Gravity provides the centripetal force needed to continually change the direction of the planet's velocity, producing a curved path.
3. Even without heavy mathematics, connect these ideas:
   1. Stronger gravity (closer to the Sun) generally leads to faster orbital motion.
   2. Larger orbital radius generally means a longer period (years are longer farther from the Sun).
   3. Elliptical orbits mean distance (and speed) can vary during the orbit.